Method for heating a seat

ABSTRACT

The invention relates to a method for heating a seat comprising a heating element (2) connected to a control unit (1), which is arranged to feed a current (I) through the heating element (2) comprising the detection of the current temperature (T) in connection to the heating element (2) and the control of the temperature (T) by feeding said current (I) through the heating element (2) if said current temperature (T) falls below a predetermined desired temperature (T B ). The invention is characterized in that it comprises determining an additional value (.increment.T B ) of said desired temperature (T B ) and adding said additional value (.increment.T B ) to said predetermined desired temperature (T B ) in connection with said control by means of the invention. An improved temperature control for a heatable seat is provided, in particular intended for motor vehicles.

TECHNICAL FIELD

The present invention relates to a method for heating a seat, accordingto the preamble of appended claim 1. In particular, the invention can beapplied when heating electrically heatable seats in a vehicle.

TECHNICAL BACKGROUND OF THE INVENTION

For reasons of comfort and safety, electrically heatable seats are usedin modern vehicles. Both the drivers seat and the other seats can be soarranged that they can be heated by means of special heating elements inthe form of electrically conducting wires which are placed in the shapeof a heating coil in each seat. Such a heating element is normallyplaced in the back-rest and in the cushion of each seat whenmanufacturing the seat. The heating element is furthermore connected toa current feeding unit which delivers current. In this manner, theheating element can be heated to a suitable temperature.

A problem of previously known heating elements is caused by the desirefor each seat to have a carefully adjusted temperature on its surface,i.e. on that surface which is in contact with the person sitting in theseat. For this purpose, the temperature of the heating element can becontrolled by means of a temperature detector which is arranged in theclose vicinity of the heating element, and which is connected to acentral control unit. Using the temperature detector and the controlunit, the current temperature can be detected. The control unit alsocomprises current feeding circuits which, for example, can be based ontransistor or relay technology, for the feeding of current to theheating element. In this way, the central control unit is arranged tofeed a certain current to the heating element until a certain desiredvalue for the temperature is reached. The setting of this desired valuecan, for example, be done by means of fixed resistances or by means ofan adjustable potentiometer, which is adjusted by a person travelling inthe vehicle.

U.S. Pat. No. 4,700,046 discloses a control device for a heating elementin a vehicle seat. The heating element can be controlled so as reach acertain set temperature.

Using the above described control method, current can be delivered tothe heating element until the central control unit indicates that thedesired value has been reached. When this happens, feeding of thecurrent stops. This causes the heating element to successively cooldown. When the heating element has cooled so that its temperature againfalls below the desired value, current feeding to the heating elementwill be resumed. In this way, the temperature control will continue foras long as the system is operative.

Although this previously known system normally provides a reliableheating and temperature control for a vehicle seat, it has, however,certain drawbacks. One such drawback is due to the fact that the heatingelement normally is assembled in the seat of the vehicle when it ismanufactured, with the heating element being adjusted according to acertain "normal" seat, with a certain given design, upholstery, etc. Theseat, in this manner, comprises a heating element for the purpose ofheating the surface of the seat to a certain desired temperature.However, when assembling the vehicle the manufacturer might choose toequip the seat in question with, for example, a completely differentupholstery, for example a considerably much thicker upholstery than thatof said "normal" seat, i.e. an upholstery which differs from that forwhich the temperature control was originally intended. The temperaturevalue which is detected by the temperature sensor will reach the setdesired value when the heating element has reached the desiredtemperature, but since there is an abnormally thick upholstery on theseat, the temperature on the surface of the seat will be too low. Thissituation thus causes an undesired deviation of the temperature control.

In a corresponding manner, similar problems can also occur if anabnormally thin upholstery is arranged on the seat, or if the heatingelement is arranged at a distance from the surface of the seat whichdeviates from that of the "normal" seat.

SUMMARY OF THE INVENTION

A main object of the present invention is thus to provide an improvedheating of a seat in a vehicle, where the above-mentioned drawbacks havebeen eliminated. This is obtained by means of a method of the initiallymentioned kind, the characteristics of which will become evident fromappended claim 1.

The invention constitutes a method for heating a seat comprising aheating element connected to a control unit which is arranged to feed acurrent through the heating element. The invention comprises a detectionof current temperature in connection to the heating element, and thecontrol of the temperature by feeding said current through the heatingelement if said current temperature falls below a predetermined desiredtemperature. The basic principle of the invention is that it comprisesdetermining an additional value of said desired temperature. Thisadditional value is then added to said predetermined desired temperaturein connection with said control. The additional value can be positive ornegative. By means of the invention, a compensation is permitted, sothat a slightly "too high" (or "too low") value of the desiredtemperature is utilized. In this way, there is provided a control withindividual adjustment to a certain seat design.

Advantageous embodiments will become apparent from the appendeddependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will in the following be described in connection to anexample of a preferred embodiment and the appended drawings, in which:

FIG. 1 is a principal circuit diagram which shows a device according tothe present invention,

FIG. 2 in principle shows a measuring bridge which is utilized whenmeasuring temperature according to the invention,

FIG. 3 in principle shows a control sequence according to the presentinvention,

FIG. 4 schematically shows how the transfer of information according tothe invention can take place.

PREFERRED EMBODIMENTS

FIG. 1 shows the principle of a circuit diagram of a device according tothe present invention. According to the preferred embodiment, theinvention is intended to be utilized in connection with electricallyheatable seats in vehicles. The figure shows, in principle, a controlunit 1 whose internal components and connections (which will bedescribed in detail below) are shown with broken lines. The figure doesnot show all of the components of the control unit 1, but only thoseparts which are necessary for the understanding of the invention.

The control unit 1 is arranged to feed a certain current I through aheating element 2. This heating element 2 is, as such, of an essentiallyknown kind, and consists of an electrical conductor which with itselectrical resistance forms a heating coil. The heating element 2 isarranged inside a (not shown) vehicle seat, preferably in its cushion.In principle, the heating element 2 can also be placed in the back-restof the vehicle. Although the figure only shows one heating element 2, itis possible to connect several such elements to the control unit 1, forexample in the form of a separate heating element for the cushion of theseat and a heating element for the back-rest of the seat. In cases wheremore than one heating element is used, these can be connected to thecontrol unit, either in parallel or in series.

As shown in FIG. 1, the heating element 2 is connected to the controlunit 1 via two connections 3 and 4 respectively, of which the latterconnection 4 is also connected to ground via a connection in the body ofthe vehicle.

In connection to the heating element 2 there is arranged a temperaturesensor 5 which is electrically connected to the control unit 1 via theabove-mentioned grounded connection 4 and a further connection 6. Thetemperature sensor 5 preferably consists of a thermistor of the NTC("Negative temperature coefficient"), which has a temperature dependentresistance R_(T) which corresponds to the temperature T which is presentin the vicinity of the heating element 2. The detection using thetemperature sensor 5 will be described in detail below.

There is furthermore a current source 7 connected to the control unit 1via a further connection 8. The current source 7 preferably consists ofthe starting battery of the vehicle. The system also comprises an on/offswitch 9, which preferably is integrated in the ignition lock of thevehicle (not shown). The switch 9 is connected to a further connection10 of the control unit 1. The control unit 1 is arranged to be able tobe activated and thus permit heating of the heating element 2 when theswitch 9 is closed.

A resistor 11 with a predetermined resistance R_(set) is connectedbetween the connection 10, to which the switch 9 is connected, and thenot grounded connection 6, to which the temperature detector 5 isconnected. As will be described in detail below, the resistor 11 isintended to be utilized in the temperature control of the heatingelement 2.

In the following, the design and main functions of the control unit 1will be described. The control unit 1 comprises a logic part 12 whichpreferably is computer based, but which can also consist of knownelectronics circuits. The logic part 12 is connected to theabove-mentioned connections 4, 6 and 10, and is arranged to detect thecurrent temperature T of the temperature sensor 5. This detection uses ameasuring bridge of the kind which is principally shown in FIG. 2. Themeasuring bridge is of the Wheatstone-bridge kind, and comprises theresistor 11 and the temperature sensor 5, which have the resistancesR_(set) and R_(T), respectively. The measuring bridge furthermorecomprises two further resistors 13 and 14 respectively, which preferablyare integrated components in the logic part 12, but which are not shownseparately in FIG. 1. The resistors 13, 14 have the resistances R₁₃,R₁₄, respectively.

The measuring bridge furthermore comprises (as shown in FIG. 2) twoconnections, between which there is a certain voltage U. One of theseconnections corresponds to the connection 6 of FIG. 1, while the otherconnection 15 is an integrated part of the logic part 12. The logic part12 is arranged to measure the voltage U when detecting the temperature Tof the temperature sensor 5. In case of balance in the measuring bridge,i.e. when the voltage U equals zero, the single unknown resistance, i.e.the resistance R_(T), of the temperature sensor 5 can be computed usingknown formulae. In this way, the logic part 12 can compute a value ofthe resistance RT, which in turn can be converted to a value of thecurrent temperature T.

With renewed reference to FIG. 1, it can be seen that the control unit 1comprises a switching unit 16 which, depending on signals from the logicpart 12, feeds the. current I to the heating element 2. The switchingunit 16, which is connected to the above-mentioned connections 3 and 8,is preferably based on a MOSFET-transistor which, as such, is a knownsemi-conductor component, which efficiently can deliver large currentsfrom the current source 7 to the heating element 2.

The logic part 12 is thus arranged to compute a value of the currenttemperature T of the temperature sensor 5. If the temperature T fallsbelow a predetermined desired value T_(B) which corresponds to a certaindesired temperature on the surface of the seat of the vehicle, and whichin general is determined by the choice of the resistances R₁₃, R₁₄,R_(set), and by the basic resistance of the thermistor 5, the logic part12 will control the switching unit 16 to deliver the current I to theheating element 2. When the desired value T_(B) is reached, the logicpart 12 will cut off the current feeding via the switching unit 16 tothe heating unit 2.

According to the embodiment, the control unit 2 preferably comprises acommunication unit 17, which is arranged to communicate with an externalunit in the form of a central computer unit 18. The communication whichtakes place via a transmission circuit 19 will be described in detailbelow.

FIG. 3 shows a diagram which in principle explains the sequence ofevents of the temperature control according to the invention. The systemis arranged to heat a vehicle seat, and for this purpose there is apreset desired value T_(B) for that temperature which is detected by thetemperature sensor 5. This desired value T_(B) can be set in advance torfor example 35° C., and corresponds to the temperature on the surface ofa "normal seat", i.e. a previously defined kind of seat with a certaingiven construction, upholstery, etc.

Due to the above-mentioned problems regarding, for example, differencesin seat upholstery of different vehicle manufacturers, there is,according to the invention, a compensation of the desired value in sucha way that a certain addition .increment.T_(B) is added to the originaldesired value R_(B). Due to this addition .increment.T_(B), which issuitable if there is an abnormally thick upholstery or an abnormallylong distance from the heating element to the surface of the seat, acompensation takes place, so that the heating element as such is heatedto a higher temperature than would have been the case otherwise. For theuser who sits on the seat, no difference is perceived, i.e. the userperceives the "normal" temperature which corresponds to the originaldesired value T_(B).

When the system is switched on, and the temperature control starts, thelogic part 12 will control the switching unit 16 so that the current Iis fed to the heating element 2. This is indicated in FIG. 3 by a solidline 20. It is assumed here that the heating element 2 has a certaininitial temperature T₁ when the heating starts. Since the current Iflows through the heating element, its temperature will successivelyincrease, which is indicated with a curve 21 with broken lines in FIG.3.

Using a conventional control system, the current feeding to the heatingelement would have continued until the ordinary desired value T_(B) hadbeen reached, following which the current feeding would have been cutoff. As opposed to this, in accordance with the present invention, thereis a continued current feeding until the compensated desired value (i.e.T_(B) +.increment.T_(B)) is reached, following which the feeding of thecurrent I is cut off. If the temperature which is detected by thetemperature sensor 5 subsequently falls below the compensated desiredvalue T_(B) +.increment.T_(B), the feeding of the current I will beresumed.

Since the seat is successively heated by the heating element, there isalso, according to the invention, a "decay" of the additional value.increment.T_(B). This means that the additional value .increment.T_(B)successively decreases and approaches zero, which is indicated withlines and dots 22 in FIG. 3. The temperature control will then continue,so that when the measured temperature T is less than the compensateddesired value T_(B) +.increment.T_(B) (which is indicated in a somewhatexaggerated manner with the curve 23), feeding of the current I to theheating element will take place. Eventually the additional value.increment.T_(B) will equal zero, following which the control willcontinue around the initial desired value T_(B).

According to the embodiment, the additional value .increment.T_(B) ischosen depending on the length of the initial current pulse, i.e. theperiod of time t₁ which elapses from the start of the control until thecurrent I stops for the first time. We thus have

    .increment.T.sub.B =k.sub.A ×t.sub.1

where k_(A) is a constant which is determined by the seat in question,and which can be positive or negative. To be more exact, the constant kAindicates to which degree the seat used deviates from a "normal" seat,for example due to an abnormally thick upholstery. The constant k_(A)can also be chosen to a certain value if the user, for reasons ofcomfort, wishes to have an "abnormally" high initial heating of theseat. The period of time t₁, i.e. that period of time during which thefeeding of the current I has to take place in order to reach a certaindesired value, has a connection to the initial temperature T₁, whichconnection can be decided using experience.

The decay time t_(A) -t₁, i.e. that period of time during which theadditional value .increment.T_(B) decreases towards zero, is furthermoredecided by the length of the initial period of time t₁, according to

    t.sub.A -t.sub.1 =k.sub.B ×t.sub.I

where k_(B) is a further constant which is a measure of how much theseat used deviates from the above-mentioned "normal" seat. This can, forexample, be caused by the upholstery which is used for the seat. If theseat used has a relatively high initial temperature T₁, a relativelyshort initial time t₁ is necessary. This, in turn, causes a relativelyshort decay time.

As shown in FIG. 3, the feeding of the current I takes place duringcertain periods, and feeding does not take place during certainintervals t_(B), t_(C), t_(D). According to one variant of theinvention, the decay of the addition to the desired value only takesplace during these intervals t_(B), t_(C), t_(D). This causes a decaywhich depends on the initial temperature T₁. To be more exact, there is,for example in the case of a very low initial temperature T₁, a currentfeeding during relatively long periods of time, with the intervalst_(B), t_(C), t_(D) then being relatively short. This, in turn, causes aslow decay.

When controlling the temperature of a seat, the desired value T_(B) isdefined in advance. This can be done by choosing the resistors 11, 13,14, and the basic resistance of the temperature detector 5 (see FIG. 2).The user can, on his own, set a desired value using a potentiometerwhich belongs to the control unit 1 (not shown), where, for example, theresistance R₁₃ (or R₁₁ or R₁₄) can be adjusted. In a particularlyadvantageous embodiment, information regarding the desired value T_(B)can be transferred to the control unit 1 from the central computer unit18 (see FIG. 1). For this purpose, the control unit 1 comprises acommunication unit 17. Its purpose is primarily to ensure thatinformation regarding the desired value T_(B) for the temperaturecontrol of the heating element 2 is transferred to the control unit 1from the central computer unit 18, which is preferably an alreadyexisting computer in the vehicle, which computer for example is utilizedfor climate control of the vehicle, the ignition system or other similarpurposes. The transfer of information takes place via a transmissioncircuit 19, which preferably consists of an electrical cable.

As shown in FIG. 4, the transfer of information between the control unit1 and the central computer unit 18 is controlled according to a periodicsequence with a certain predetermined period t₁. The transfer ofinformation is based on the principle of transferring information whichcorresponds to a certain set value of the desired temperature T_(B) fromthe central computer unit 18 to the control unit 1. Preferably, there isfurthermore also a transfer of information in the opposite direction,i.e. from the control unit 1 to the central computer 18. The informationwhich is transmitted from the control unit 1 can, for example, comprisestatus information. During the total period of time t₁, transfer takesplace from the control unit 1 during a certain period of time t₂, whilethe transfer to the control unit 1 takes place during another period oftime t₃.

FIG. 4 thus shows a certain period for the transfer of information.According to the preferred embodiment of the invention, the transfer ofinformation is initiated by a start-bit 24 being transferred from thecontrol unit 1 to the central computer unit 18. For this purpose, thecommunication unit 17 comprises a (not shown) oscillator circuit which,as such is known, and which is arranged to be able to periodically emitpulses via a connection 19. The central computer unit 18 furthermorecomprises a detection circuit (not shown), which, as such is known, forthe detection of pulses via the connection 19. The transfer of astart-bit 24 from the control unit 1 initiates a certain period, andindicates that the control unit 1 is ready for function, and thatfeeding of current to the heating element 2 can take place.

Subsequent to the transfer of the start-bit 24 there is, where used, atransfer of one or two status bits 25, 26, respectively from the controlunit 1. According to the embodiment, the first status-bit 25 will betransferred if the heating element 2 is "active", i.e. if there isfeeding of current to the heating element 2. In this case, a negativepulse is thus transferred, as indicated in the drawing. The secondstatus-bit 26 will furthermore be transferred (in the form of a negativepulse) if there is a malfunction in the heating element 2. Examples ofmalfunctions which might occur are that some part of the heating element2 has been short-circuited, or that the conductor which constitutes theheating element 2 has been broken. The start-bit 24 and the twostatus-bits 25, 26, can thus be transferred during a time t₂ which isdefined in advance, and thus deliver information regarding the currentstatus of the heating element 2 to the central computer 18. The transferof the bits 24, 25, 26 is asynchronous, i.e. the pulses are counted bythe central computer unit 18, which thus is the receiving side.

The next phase of the transfer is the transfer of a desired value T_(B)for temperature control of the heating element 2. This desired value istransferred from the central computer 18 to the control unit 1 duringthe period of time t₃. To be more exact, the transfer takes place viathe connection 19 and the communication unit 17 to the logic part 12(see FIG. 2). During the period of time t₃ there is thus a transfer of anumber of pulses 27 from the central computer 18. For this purpose, thecommunication unit 17 is also equipped with a (not shown) detectioncircuit for counting the number of pulses 27. The number of pulses 27preferably corresponds to a certain desired value T_(B) for temperaturecontrol of the heating element 2. By way of example, FIG. 3 shows fivepulses 27 being transferred. This might correspond to a desired valueT_(B) which, for example, can amount to 35° C., which in turncorresponds to a certain desired temperature on the surface of the seat.If, for example, a desired value of 36° C. is desired, six pulses 27 canfor example be transferred.

According to the invention, the central computer unit 18 can alsocompute the addition .increment.T_(B) to the desired value and the decaytime t_(A) -t₁. This can be done using algorithms in the software of thecomputer unit 18. The information which is transferred by means of thepulses 27 is based on these values regarding the addition.increment.T_(B) to the desired value and the decay time t_(A) -t₁.

A compensated desired value T_(B) +.increment.T_(B) can thus betransferred to the logic part 12. With reference to FIGS. 1 and 2, itcan now be seen that a given desired value corresponds to a certainexpected resistance R_(T) of the temperature sensor 5. This correspondsto the logic part 12 changing the values of the resistances R₁₃ and R₁₄,which cause balance in the measuring bridge (see FIG. 2) at the currentdesired temperature. This can be done by (not shown) switch transistorsin the logic part 12 switching between different resistance values in aresistance ladder. The resistance R_(set) is not affected by whichdesired value T_(B) is transferred. When the correct temperature hasbeen reached, the resistance R_(T) of the temperature sensor 5 will beof such a magnitude that balance is reached in the measuring bridge.This corresponds to the desired value (which thus can be a compensateddesired value) having been reached.

Since the central computer 18 can deliver information regarding desiredvalues, a correct control of the heating element 2 is obtainedregardless of, for example, the upholstery of the seat used. The centralcomputer can, already when manufacturing the vehicle, be provided withinformation regarding which seat is used, which in turn givesinformation regarding current desired values (T_(B), .increment.T_(B))decay times (t_(A) -t₁) and constants (k_(A), k_(B)).

According to the embodiment, the logic unit 12 is arranged to alsodetect the case where no pulse 27 at all is transferred during theperiod of time t₃. This is interpreted as a "reset" signal by the logicpart 12, and causes any ongoing current feeding to the heating elementto cease. The entire logic part 12 is furthermore also set to zero, i.e.flip-flops, switches, registers and counters are set to zero.

For example, error flip-flops which detect short-circuits in the heatingelement are set to zero. In this way, intermittent malfunctions can bedetected. Preferably, the entire system is also shut off, so that anyheating ceases if there is reception of too large a number of pulses,i.e. a number of pulses which exceeds the highest desired temperature.

Preferably, the invention utilizes an increase time or "step-in" time,during which the additional value .increment.T_(B) increasessuccessively from zero to the determined value. In case of theadditional value AT_(B) being negative, there is instead a successivedecrease from zero during the step-in time. In a corresponding manner a"step-out" time is also utilized, during which the additional value.increment.T_(B) successively approaches zero. This can, for example,take place with a predetermined step time subsequent to the heatingelement having been shut off when the compensated desired value (T_(B)+.increment.T_(B)) has been reached for the first time, in which casethe desired value for the temperature control resumes the initialdesired value T_(B).

It is furthermore an essential object of the invention to eliminate, asfar as possible, the thermal pumping effect which can occur in the formof large temperature variations on the surface of the seat used, andwhich are caused due to current switched on and switched of f, and whichalso depend on the current distance between the heating element 2 andthe temperature sensor 5. For this purpose, the invention can, accordingto a particular embodiment, utilize so-called proportional band control,which as such is a previously known kind of control, which is utilizedhere so that, within a certain temperature band, the effect from theheating element is varied proportionally in relation to the temperaturewithin the band. The variation in effect which is carried out can belinearly or non-linearly proportional to the current temperature withinthe band. At temperatures above the chosen band, the current feeding tothe heating element is cut off completely, and at temperatures which arebelow the chosen band there is a maximal current feeding to the heatingelement.

The temperature band utilized in the proportional band control is chosendepending on the kind of seat, upholstery, padding and other relevantproperties of the materials of the seat.

When transferring information regarding the desired value during saidproportional band control, this information can be varied periodicallywith a predetermined frequency. The time of this period is chosen sothat no thermal pumping effect is obtained in the seat. For a normalseat, a frequency is chosen which corresponds to a period which isshorter than about 15 seconds, preferably about 1-2 seconds. Thefrequency (as well as the effect) can also be varied with the time, orwith the length of the "initial pulse" which occurs during heating untilthe heating element is shut off for the first time, subsequent to acompensated desired value having been reached.

The desired values which are transferred to the control unit 1 duringthe proportional band control are preferably pulses which vary accordingto a certain pattern, and with a certain frequency. The temperature bandwhich is chosen for the control is preferably a certain range, which is(symmetrically or asymmetrically) positioned around the change-overpoint which corresponds to the compensated desired value (i.e. T_(B)+.increment.T_(B)). The temperature band chosen (i.e. its upper andlower boundary value) can also be made to vary in time when controlled.With a properly chosen pattern and frequency in the proportional bandcontrol, in principle no temperature variation is obtained on thesurface of the seat.

During proportional band control, the effect can furthermore be variedlinearly or non-linearly within the current temperature band. In case ofnon-linear variation, the degree of non-linearity can be varied with thelength of the above-mentioned initial pulse.

In order to limit the temperature variations on the surface of the seat,the invention can alternatively be equipped with effect limits. Thiswould here mean that, subsequent to the first shutting off of thecurrent feeding to the heating element (subsequent to the compensateddesired value T_(B) +.increment.T_(B) having been reached), the currentfeeding to the heating element will be shut off and turned on with arelatively high frequency and with a limited effect. In this way, thetemperature variations on the surface of the seat can be decreased.During such limiting of the effect, the effect can be varied linearly ornon-linearly. The frequency with which the heating element is connectedand disconnected can also be varied in time. This frequency can,furthermore, vary depending on the above-mentioned "initial pulse".

As can be understood from the above description, the communicationbetween the central computer unit 17 and the control unit 18 is of theserial kind. This means that only one connection is necessary betweenthe central computer unit 17 and the control unit 1, which in turnreduces the costs in connection with the invention.

The duration of the periods of time t₁, t₂ and t₃, can be varied, anddepends on how the oscillator circuit in the communication unit 17 isdesigned. Preferably a period-length t₁ is used (i.e. the time betweentwo start pulses 23) which is of the order of size 600 to 1000 ms; theperiod of time t₂ is about 100-200 ms and the period of time t₃ is about500-800 ms. In this way, the period of time t₂ constitutes approximately10-30% of the total period of time t₁, while the period of time t₃constitutes about 70-90% of an entire period. The central computer unit18 detects the start of a certain period by detecting the start-bit 24.The computer unit 18 can also compute the period of time t₁ by measuringthe time which elapses between two start-bits 24. By knowing duringwhich part of the period of time t₁ information regarding the status ofthe heating element is expected to be received, the pulses 25 and 26 canbe detected. Subsequent to this, a certain number of pulses 27 can betransferred during the period of time t₃.

The components of the control unit 1 can, using modern technology, beintegrated into one single application specific integrated circuit(ASIC), which provides a very high reliability of the invention. In sodoing, the communication unit 17, the logic part 12 and the switchingunit 17 are preferably arranged on the same silicon chip. Alternatively,the various circuits can be assembled on separate silicon chips but inthe same circuit, i.e. in the same package.

The invention is not limited to that which has been described above:various embodiments are possible within the scope of the claims. Theinvention can, for example, in principle be utilized to heat other seatsthan vehicle seats. Different kinds of temperature sensors canfurthermore be utilized, for example thermistors with a negative orpositive temperature coefficient. An existing temperature sensor in thevehicle can also, in principle, be used. The switching unit 16 canfurthermore be based on, for example, MOSFET or relay technology.

The additional value .increment.T_(B) can furthermore be negative aswell as positive.

When heating a seat, a warm seat requires less energy than a cold seatin order to be heated. This means that there is a connection between thevalue of the period of time t₁, the initial temperature T₁ and thedesired temperature, which can be used for control purposes. The initialtime t₁ can, for example, give a value of the initial temperature T₁.

It should be noted that the invention can also be utilized if no startand status information (i.e. bits 24, 25 and 26) is transferred from thecontrol unit 1. This corresponds to the existence of one-waycommunication from the central computer unit 18 to the control unit 1.The minimum of information which must be transferred from the centralcomputer unit 18 is a series of pulses 27, which is transferred within acertain interval of time and which indicates a certain desired value forthe temperature control. Said desired value furthermore does not need tobe transferred in such a way that the number of pulses determines acertain temperature value. Coded signals can instead be transferred,where a certain digital word corresponds to a given temperature value.

If status information is transferred from the control unit 1, the numberof status bits does not necessarily need to be two, but can be varieddepending on the information which is intended to be transferred fromthe control unit 1.

According to the invention, the information regarding the current valueof the desired value can vary during the temperature control, inparticular during the initial pulse which lasts until the heatingelement is turned off for the first time, subsequent to a compensateddesired value having been reached. The length of the initial pulse canbe linked to a variety of different parameters in connection with thecontrol, for example the "step-in" time, the "step-out" time, and thesize and frequency of the proportional band control. The period of timeduring which heating with the heating element takes place (or,alternatively, the time when heating does not take place) can also belinked to the corresponding parameters.

The current desired value which is used in the control can, according toan alternative embodiment, follow a predetermined relationship whichvaries in time. In this way, a compensation of the desiredcharacteristics on the surface of the seat which is used is enabled.

It is furthermore not necessary to use communication with an externalunit 18 in order to provide the compensated and dynamically varyingdesired value T_(B) +.increment.T_(B). If an external computer unit isnot utilized, this information can, for example, be determined using anRC-circuit in the logic part 12 whose time constant in controlled by thecurrent feeding pulses.

Finally the connection 19 can consist of an electrical cable, an opticalcable or a radio connection.

What is claimed is:
 1. Method for heating a seat comprising a heatingelement (2) connected to a control unit (1), which is arranged to feed acurrent (I) through the heating element (2), comprising:detection of thecurrent temperature (T) of the heating element (2), control of thetemperature (T) by feeding said current (I) through the heating element(2) if said current temperature (T) falls below a predetermined desiredtemperature (T_(B)), the control of the temperature (T)comprising:determining an additional value (.increment.T_(B)) for saiddesired temperature (T_(B)), the magnitude of said additional value(.increment.T_(B)) being dependent on the operation or the design ofsaid seat, and adding said additional value (.increment.T_(B)) to saidpredetermined desired temperature (T_(B)) in connection with saidcontrol for adjusting the temperature (T) of the heating element (2)during said heating of the seat so that the temperature of the surfaceof the seat corresponds to said predetermined desired temperature(T_(B)).
 2. Method according to claim 1, wherein information (27)regarding said desired temperature (T_(B)) or said additional value(.increment.T_(B)) is transferred to said control unit (1) from anexternal unit (18).
 3. Method according to claim 2, wherein information(25, 26) regarding the status of said heating element is alsotransferred from said control unit (1) to said external unit (18). 4.Method according to claims 2 or 3, wherein said information istransferred serially via a transmission channel (19) between the controlunit (1) and the external unit (18).
 5. Method according to claims 2 or3, wherein said information is transferred in the form of a series ofpulses, the number of which directly corresponds to a value of saiddesired temperature (T_(B)) and/or said additional value(.increment.T_(B)).
 6. Method according to claims 2 or 3 wherein saidmethod comprises a proportional band control, with said transferredinformation (27) being chosen so that the effect from the heatingelement (2) is varied periodically and proportionally to the currenttemperature (T) if the current temperature (T) is within a predeterminedinterval.
 7. Method according to claim 2 or 3, further comprisingswitching, with the feeding of the current (I) to the heating element(2) being switched on and off with a predetermined frequency and effect,subsequent to a predetermined desired temperature (T_(B)+.increment.T_(B)) having been reached.
 8. Method according to claim 1,wherein said additional value (.increment.T_(B)) is determined by meansof an RC-circuit in the control unit (1).
 9. Method according to claim1, wherein said control is initiated by current (I) being fed throughsaid heating element (2) until a value which corresponds to the sum ofsaid additional value (.increment.T_(B)) and said desired temperature(T_(B)) is reached, subsequent to which the current feeding ceases, andwith the additional value (.increment.T_(B)) being calculatedproportionally according to the initial time (t₁) which has elapseduntil the current feeding has ceased.
 10. Method according to claim 1,wherein said additional value (.increment.T_(B)) is reduced to zeroduring a decay time (t_(A) -t₁).
 11. Method according to claim 10,wherein said decay time (t_(A) -t₁) is decided depending on the initialtime (t₁) which has elapsed until the current feeding has ceased after avalue which corresponds to the sum of said additional value(.increment.T_(B)) and said desired temperature (T_(B)) is reachedduring control being initiated by current (I) being fed through saidheating element (2).
 12. Method according to claim 10, wherein saiddecay time (t_(A) -t₁) is decided in proportion to a value (k_(B)) whichconstitutes a measure which corresponds to the design of said seat. 13.Method according to claim 10, wherein said reduction of the additionalvalue (.increment.T_(B)) takes place during periods of time (t_(B),t_(C), t_(D)) during which said current feeding does not take place. 14.Method according to claim 1 wherein said additional value(.increment.T_(B)) is increased, or decreased, from zero to its chosenvalue (.increment.T_(B)) during a predetermined increase time. 15.Method according to claim 1 wherein said additional value(.increment.T_(B)) is determined proportionally to a value (k_(A)) whichconstitutes a measure corresponding to the design of said seat.